In order to achieve a high efficiency, a high turbine inlet temperature is required in standard gas turbines. As a result, there arise high NOx emission levels. These emissions can be mitigated with a sequential combustion. The main flow passes the first combustion chamber (e.g. using a burner of the general type as disclosed in EP 1 257 809 or as in U.S. Pat. No. 4,932,861, also called EV combustor, where the EV stands for environmental), wherein a part of the fuel is combusted. The remaining fuel is added and combusted (e.g. using a burner of the type as disclosed in U.S. Pat. No. 5,431,018 or U.S. Pat. No. 5,626,017 or in US 2002/0187448, also called SEV combustor, where the S stands for secondary). Both combustors contain premixing burners, as low NOx emissions require high mixing quality of the fuel and the oxidizer.
Since the second combustor is fed by exhaust gas of the first combustor (potentially cooled with some additional cooling air), the operating conditions allow self-ignition (spontaneous ignition) of the fuel air mixture without additional energy being supplied to the mixture. To prevent ignition of the fuel air mixture in the mixing region, the residence time therein must not exceed the auto ignition delay time. This criterion ensures flame-free zones inside the burner. This criterion poses challenges in obtaining appropriate distribution of the fuel across the burner exit area.
The subsequent mixing of the fuel and the oxidizer at the exit of the mixing zone is just sufficient to allow low NOx emissions (mixing quality) and avoid flashback (residence time), which may be caused by auto ignition of the fuel air mixture in the mixing zone.
Current designs are limited due to the relation of burner exit area, flow velocity and post injection residence time in the burner. This poses difficulties in achieving circumferential mixing.